This invention relates to a circuit arrangement for operating a lamp comprising
circuit input terminals for connection to a supply voltage source, an inverter coupled to said circuit input terminals for generating an AC voltage with a frequency f from a supply voltage supplied by the supply voltage source and equipped with inverter output terminals, PA1 a piezotransformer comprising transformer input terminals, coupled to the inverter output terminals, and transformer output terminals, PA1 terminals for lamp connection coupled to the inverter output terminals, and PA1 a detector for detecting whether the lamp has ignited.
Such a circuit arrangement is known from Jp H6-89789. The known circuit arrangement is very suitable for operating a discharge lamp and comprises a switching circuit part that connects the inverter output to the transformer input during the ignition of the lamp. The lamp is directly connected to the transformer output terminals. The frequency f of the AC voltage generated by the inverter is very close to one of the resonance frequencies of the piezotransformer. Therefore the AC voltage is transformed by the piezotransformer to an ignition voltage with the same frequency f but a much higher amplitude that is present across the lamp. After the lamp has ignited under the influence of this ignition voltage, the detector which is part of the circuit arrangement, detects a lamp current and generates a signal that triggers the switching circuit part to disconnect the inverter output and the transformer input. After the transformer input has been disconnected from the inverter output, the piezotransformer no longer generates the ignition voltage and the lamp is operated by means of the AC voltage at the frequency f that is generated by the inverter. An important advantage of the known circuit arrangement is that the inverter is used both in the generation of the voltage that is used to operate the lamp during stationary conditions as well as in the generation of the ignition voltage. For this reason the known circuit arrangement comprises a relatively small amount of components and is therefore relatively inexpensive and compact. A disadvantage of the known circuit arrangement, however, is that during ignition the high ignition voltage is present between the output terminals. This can easily lead to damage to the inverter.